Statistical scaling of soft decisions before decoding

ABSTRACT

A wireless-communications unit has a receiver chain including a codec decoding the output of a rake receiver. Soft decisions passing from the rake receiver to the codec are scaled. The scaling factor can be tuned by monitoring a function based on the cumulative probability distribution function of the scaled soft decisions.

The invention relates to the field of wireless-communications networks.For example, the invention finds application in a network organisedaccording to the UMTS (Universal Mobile Telecommunications System)standards. In particular, the invention is concerned with the receiverarchitecture of a participant in a wireless-communications network, suchas a base station or a mobile telephone.

FIG. 1 shows a block diagram of a receiver chain for a UMTS mobiletelephone 10. Radio frequency signals transmitted to the telephone 10are received on antenna 12 and passed to demodulator 14. In thedemodulator 14, spread-spectrum signals are extracted from their radiofrequency carrier signals and are sent to a digital to analogueconverter (DAC) 16. The digitised spread-spectrum signals are then sentto a rake receiver 18. The rake receiver identifies multi-pathcomponents within the spread-spectrum signals, despreads the multi-pathcomponents and combines them to produce an encoded signal at the chiprate consisting of a series of soft decisions. The encoded signal isthen operated on by scaling unit 20 which applies a scaling factor tothe soft decisions constituting the encoded signal. The scaled versionof the encoded signal output by the scaling unit 20 is then passed to acodec 22 (an encoder-decoder). The codec 22 decodes the encoded signalusing a process such as the Viterbi algorithm. The codec 22 outputs asymbol-rate message signal as a result of the decoding process. Themessage signal then undergoes further processing within the telephone10, as indicated by processor block 24. For example, the message signalcould indicate digitised speech which requires additional processing toconvert it into an analogue signal which can be emitted from aloudspeaker (not shown) forming part of the telephone 10.

As mentioned above, the scaling unit 20 scales the soft decisions thatare produced by the rake receiver 18 for decoding in the codec 22. Acertain degree of scaling is required to optimise the operation of thecodec 22. The scaling unit 20 controls the degree of scaling that itapplies to the soft decisions by monitoring its own output on feedbackpath 26. That is to say, the scaling unit 20 monitors the scaled softdecisions that it sends to the codec 22 and uses the monitored softdecisions in an algorithm which determines how the degree of scalingshould be adjusted.

The details of the operation of the algorithm are not important and canvary from one implementation to another. However, the algorithm willproduce an output in the form of an adjustment to be made to the scalingfactor that is applied to the soft decisions and this output is derivedby monitoring the power of the soft decisions arriving on the feedbackpath 26. A significant disadvantage of this approach will now bediscussed.

FIG. 2 is a plot of a digital signal with amplitude and time representedon the vertical and horizontal axes, respectively. In FIG. 2, the signalcomprises a series of digital samples, issued at a rate of one sampleper unit of time. The power, P, contained in the signal shown in FIG. 2is simply the sums of the squares of the amplitudes of the individualsamples making up the signal, divided by the number of samples:S=0²+1²+2²+3²+2²+1²+0²+1²+2²+3²+2²+1²+0²+1²+2²+1²+0²+1²45P=45/18=2.5

FIG. 3 shows what would happen if the signal of FIG. 2 became saturatedat an amplitude of 2. The power of the saturated signal can also becalculated:S=0²+1²+2²+2²+2²+1²+0²+1²+2²+2²+2²+1²+0²+1²+2²+1²+0²+1²=35P=35/18=1.9

It will be seen that when the signal becomes saturated at an amplitudeof two, the power value is drastically different to the power value thatis obtained when the signal is not saturated. Therefore, in situationswhere the signal might become saturated, the value of the power of thesignal might not be a good parameter to monitor.

Sometimes, in the receiver chain shown in FIG. 1, it is desirable to usea certain form of decoding algorithm within the codec 22, this algorithmbeing of a kind whose accuracy is increased if the signal that itoperates on is partially saturated. Therefore, the situation can arisewhere the scaling unit 20 is intentionally inducing partial saturationof the soft decisions provided by the rake receiver 18. In thissituation, a problem arises if the algorithm used to scale the softdecisions relies on the power of the soft decisions that are monitoredon feedback path 26. That is to say, because the power value for anygiven signal monitored on feedback path 26 can vary tremendouslydepending upon the degree of scaling being applied to the signal, thepower of the signal monitored on path 26 is not always a reliableparameter to use for controlling the algorithm which scales the softdecisions.

One aim of the present invention is to ameliorate, at least in part, thedisadvantage discussed above.

According to one aspect the invention provides a method of conditioningsignal values being conveyed to a decoder in a wireless-communicationsnetwork participant, the method comprising scaling the values,monitoring the probability distribution of the amplitudes of the scaledvalues and using the information gained through the monitoring step todetermine if the degree of scaling should be adjusted.

The invention also consists in a wireless-communications networkparticipant, comprising a decoder for decoding a signal received at theparticipant, scaling means for scaling values of the signal beingconveyed to the decoder, monitoring means for monitoring the probabilitydistribution of the amplitudes of the scaled values and control meansfor using information supplied by the monitoring means to determine ifthe degree of scaling should be adjusted. Such a method can beimplemented by as a computer program. Such programs can be accommodatedby an appropriate data carrier, such as a read only memory.

Thus, the invention provides an alternative scheme for scaling a streamof signal values which is less effected by saturation.

In one embodiment, the probability distribution of the scaled values ismonitored by determining the fraction of a group of signal values thatexceed a certain amplitude.

The invention can be used, for example, in a participant of a 3Gtelecommunications network and the decoder is a bit-rate signal decoderusing, for example, the Viterbi or max log-MAP algorithm.

By way of example only, an embodiment of the present invention will nowbe described with reference to the accompanying figures, in which:

FIG. 1 is a block diagram of a receiver chain in a UMTS mobiletelephone;

FIG. 2 is a plot of a group of soft decisions representing a section ofa signal;

FIG. 3 is a plot of the signal of FIG. 2 in a partially saturatedcondition;

FIG. 4 is a plot of the amplitude of the soft decisions of FIG. 3;

FIG. 5 illustrates the performance of a probability distributionmeasurement on the wave form of FIG. 4; and

FIG. 6 illustrates the performance of a probability distributionmeasurement on a trace representing the amplitude of the signal shown inFIG. 2.

In the embodiment that will now be described, the power of the signal onpath 26 is not the parameter which is used to control the algorithmwhich scales the soft decisions. Instead, a parameter based on thecumulative probability density function (CDF) of the signal is used. Infact, the parameter that is used to control the scaling algorithm is1-CDF and is hereinafter termed the complementary CDF and is referred toas Q for ease of reference.

The CDF for a given amplitude value of a digital signal is the ratio ofthe number of samples in the signal where the amplitude is equal to orless than the given value versus the total number of samples in thesignal. The parameter Q is the ratio of the number of samples where theamplitude value exceeds the given value versus the total number ofsamples in the signal. That is to say, CDF+Q=1.

To illustrate the calculation of the parameter Q, we will use theexample of the saturated signal of FIG. 3. FIG. 4 is a plot of theamplitude of the saturated signal of FIG. 3 versus time. In thisexample, we will calculate Q [1], which is to say the ratio of thenumber of samples where the amplitude exceeds 1 versus the total numberof samples.

FIG. 5 illustrates this calculation. As can be seen in FIG. 5, theamplitude of the signal exceeds the value 1 on 7 occasions (as indicatedby the vertical arrows) out of the 18 amplitude points shown. Therefore,Q [1]=0.389 for the signal shown in FIG. 5.

FIG. 6 shows what the calculation of FIG. 5 would have been like if thesignal had not been saturated. Again, the amplitude of the signalexceeds the value 1 on 7 occasions (as indicated by the vertical arrows)out of the 18 sample points. Therefore, the result for Q [1] isunchanged by saturation in this example.

Therefore, it follows that CDF, or CDF based measurements, can be usedto make an assessment of the amplitude profile of a signal without beinggrossly effected by saturation. Of course, the degree of effect thatsaturation has on a CDF or Q value depends on the amplitude value atwhich saturation occurs relative to the amplitude at which the CDF or Qfunction is evaluated. For example, if the amplitude value of saturationis less than the amplitude value at which Q is evaluated, then the Qvalue will be significantly distorted by saturation. Therefore, it isimportant to keep the amplitude value at which the Q function iscalculated to a level below the saturation limit.

Returning to the embodiment, with the exception of scaling unit 20, theoperation of the receiver chain is much the same as that describedearlier with reference to FIG. 1. In the embodiment, the scaling unitbetween the rake receiver and the codec evaluates the soft decisions onthe feedback path by determining their Q function value at a selectableamplitude level.

It is worth bearing in mind, as will be readily apparent to the skilledperson, that the waveforms shown in FIGS. 2 to 6 are for the purposes ofillustration only and that UMTS signals have a different appearance.

1-11. (canceled)
 12. A method of conditioning signal values beingconveyed to a decoder in a wireless-communications network participant,the method comprising scaling the values, monitoring the probabilitydistribution of the amplitudes of the scaled values and using theinformation gained through the monitoring step to determine if thedegree of scaling should be adjusted.
 13. A method according to claim12, wherein the monitoring step comprises calculating a complementarycumulative probability density function for a signal value magnitude.14. A method according to claim 12, wherein the monitoring stepcomprises determining the fraction of a group of signal values thatexceed a certain magnitude.
 15. A method according to claim 12, whereinthe decoder is a 3G telecommunications bit-rate signal decoder.
 16. Awireless-communications network participant, comprising a decoder fordecoding a signal received at the participant, a scaler adapted to scalevalues of the signal being conveyed to the decoder, a monitor adapted tomonitor the probability distribution of the amplitudes of the scaledvalues and a controller adapted to use information supplied by themonitor to determine if the degree of scaling should be adjusted.
 17. Aparticipant according to claim 16, wherein the monitor is adapted tocalculate a complimentary cumulative probability density function for asignal value magnitude.
 18. A participant according to claim 16, whereinthe monitor is adapted to determine fraction of a group of signal valuesthe exceed a certain magnitude.
 19. A participant according to claim 16,wherein the decoder is a 3G telecommunications bit-rate signal decoder.20. A data carrier containing programming code adapted to cause dataprocessing apparatus to carry out a method of conditioning signal valuesbeing conveyed to a decoder in a wireless-communications networkparticipant, the method comprising scaling the values, monitoring theprobability distribution of the amplitudes of the scaled values andusing the information gained through the monitoring step to determine ifthe degree of scaling should be adjusted.